When MrCjolsen says "suspension corrected" he is referring to the fact that suspension forks are typically taller, axle to crown, than non-suspension forks. They may also have less rake.
Bike frames are designed for a specific fork height and rake. If the new fork is shorter you end up with steeper headtube and seattube angle and a lower bottom bracket. This can be good or bad, but if you want similar handling with the new fork you need to measure the current forks axle to crown height minus sag.
To measure sag I've generally heard you should place a zip tie, snug enough that it will stay in place, at the bottom of one stanchion, ride the bike a short distance on a flat surface, get off, and measure how high the tie is.